PEARSON CHEMICAL HARDNESS, HETEROLYTIC DISSOCIATIVE VERSION OF PAULING BOND-ENERGY EQUATION AND A NOVEL-APPROACH TOWARDS UNDERSTANDING PEARSON HARD-SOFT ACID-BASE PRINCIPLE

The empirical chemical hardness parameters developed recently by Pearson have been examined critically. It was found that these parameters normally characterise a group but not the corresponding ion as postulated. Equation (i) has been found quite successful when the heterolytic dissociation of D(A+B-) - 1/2[D(A+A-) + D(B+B-)] = 2\DELTA-gamma\2 (i) AB into A+ + B- follows the general notion of electronegativity (i.e. the more electronegative part of AB dissociates as the anion), where D(A+B-) is the energy required to dissociate the AB bond into A+ + B- and D(A+A-) and D(B+B-) are the heterolytic bond-dissociation energies of the molecules AA and BB respectively. Equation (i) has been used to define a new parameter gamma, where \DELTA-gamma\ = \gamma-A+ - gamma-B-\. The gamma parameters characterise the ions, and have been used to rank various monovalent anions and cations in a unified way to explain the hard-soft acid-base principle of Pearson. It was found that a reaction of type (ii) proceeds from left to right in the gas phase if the AB + CD ----> AC + BD (ii) \DELTA-DELTA-gamma\ value of the right-hand side is greater than that of the left. Out of some 282 gas-phase reactions considered, there are few exceptions. Emphasis has been placed on explaining the course of inorganic reactions. It is felt that the gamma parameter can be identified with the chemical hardness of an ion.